1. Field of the Invention
The present invention relates to a confocal displacement sensor in which light in a wide wavelength band is used.
2. Description of Related Art
As a device that measures displacement of the surface of a measurement object in a noncontact manner, there is known a confocal displacement sensor. For example, JP-A-2013-130581 (Patent Literature 1) describes a chromatic point sensor (CPS) system that measures the distance from a predetermined reference position to a measurement object as displacement of the surface of the measurement object. The CPS described in Patent Literature 1 includes two confocal-system optical paths. Lights having a plurality of wavelengths are input to the optical paths. The light passed through either optical path is selectively output to the measurement object.
A first optical path is configured such that lights having different wavelengths are focused at different distances near a surface position of the measurement object in an optical axis direction. The light passed through the first optical path is reflected on the surface of the measurement object. In the reflected light, only light focused on the position of an opening section disposed in the first path as a spatial filter passes through the opening section to be guided to a wavelength detector. A spectrum profile (a first output spectrum profile) of the light detected by the wavelength detector includes a component indicating a measurement distance (distance-dependent profile component) and includes a distance-independent profile component.
A second optical path is configured such that lights having different wavelengths are focused at substantially the same distance near the surface position of the measurement object. The light passed through the second optical path is reflected on the surface of the measurement object. In the reflected light, only light focused on the position of an opening section disposed in a second path as a spatial filter passes through the opening section to be guided to a wavelength detector. A spectrum profile (a second output spectrum profile) of the light detected by the wavelength detector does not include a distance-dependent profile component and includes only a distance-independent profile component.
Correction for a potential measurement error related to the distance-independent profile component is performed concerning the first output spectrum profile using the second output spectrum profile.
In the CPS system described in Patent Literature 1, since the correction is performed concerning the first output spectrum profile, reliability is improved. Specifically, a measurement error due to, as the distance-independent profile component, a material component of the measurement object, a spectrum profile component of a light source associated with the light source, or a component associated with the wavelength detector is reduced. However, in the confocal displacement sensor, a measurement error in a degree larger than the roughness of the surface occurs because of the influence of irregular reflection on the surface of the measurement object. In the CPS system described in Patent Literature 1, such a measurement error cannot be reduced.